Human and rabbit platelets contain platelet microbicidal proteins (PMPs). The investigators' data show that PMPs play key roles in platelet antimicrobial functions. PMPs exert potent microbicidal actions against antibiotic-resistant bloodstream pathogens, including Staphylococcus aureus and Candida albicans. Specific PMPs are released from platelets exposed to pathogens or agonists at sites of endovascular infection, and intensify locally at these sites. PMP-susceptible pathogens are less virulent in animal models than their isogenic PMP-resistant counterparts. These compelling facts support their hypothesis that PMPs significantly contribute to antimicrobial host defense. PMPs are minimally toxic to human vascular endothelial cells or erythrocytes, and differ markedly in structure and mechanism from antimicrobial peptides that are not released into the bloodstream. These facts suggest PMPs have key functional determinants that optimize microbicidal activity without concomitant host cytotoxicity. Beyond its microbicidal effects, they have discovered that PMP-2 also potentiates neutrophil chemotaxis, phagocytosis and intracellular killing of S. aureus. PMP-2 has a cystine-X-cystine (CXC) motif distinctive of alpha-chemokines such as human platelet factor-4 (hPF-4) that amplify antimicrobial mechanisms of neutrophils. These facts indicate that PMP-2 is a unique molecule that exerts both direct microbicidal and neutrophil-potentiating effects. The investigators' central hypothesis contends that PMP-2 has specific determinants responsible for these distinct host defense functions. They further hypothesize these determinants can be defined, modeled, and used to establish key structure-activity relationships (SARs) governing specific functions. The proposed studies are designed to explore these hypotheses. Defining SARs in PMP-2 functional determinants is crucial to their eventual goal of designing anti-infective agents with potent and/or selective activity against antibiotic-resistant pathogens. Therefore, their Specific Aims are: i) to define the structural determinants responsible for direct microbicidal functions of PMP-2; ii) define the PMP-2 structural determinants that potentiate the antimicrobial functions of neutrophils; and iii) establish the key SARs in antimicrobial determinants of PMP-2. Comparison of PMP-2 and hPF-4 determinants responsible for their potent and/or discriminative antimicrobial functions will enable future studies of human PMPs in the rabbit model that cannot be conducted in humans. Moreover, SAR themes discovered in PMP-2 will accelerate discovery of novel anti-infective strategies against pathogens resistant to conventional agents. Thus, these studies will significantly advance our understanding of antimicrobial host defense, and may yield new modes for its amplification.